In order to estimate photosynthetic and respiratory rates in illuminated photoautotrophic cells of carnation (Dianthus caryophyllusL.), simultaneous measurements of CO2and O2gas exchange were performed using18O2,13CO2and a mass-spectrometry technique. This method allowed the determination, and thus the comparison, of unidirectional fluxes of O2and CO2. In optimum photosynthetic conditions (i.e. in the presence of high light and a saturating level of CO2), the rate of CO2influx represented 75±5 of the rate of gross O2evolution. After a dark-to-light transition, the rate of CO2efflux was inhibited by 50 whereas the O2-uptake rate was little affected. The effect of a recycling of respiratory CO2through photosynthesis on the exchange of CO2gas was investigated using a mathematical model. The confliction of the experimental data with the simulated gas-exchange rates strongly supported the view that CO2recycling was a minor event in these cells and could not be responsible for the observed inhibition of CO2efflux. On the basis of this assumption it was concluded that illumination of carnation cells resulted in a decrease of substrate decarboxylations, and that CO2efflux and O2uptake were not as tightly coupled in the light as in the dark. Furthermore, it could be calculated from the rate of gross photosynthesis that the chloroplastic electron-transport chain produced enough ATP in the light to account for the measured CO2-uptake rate without involving cyclic transfer of electrons around PS I or mitochondrial supplementation
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